In many countries it is compulsory for vehicles which are used on public roads and thoroughfares to have light reflectors mounted on the rear of the vehicles. In addition to the standard reflectors, most heavy vehicles and trailers have light reflecting chevrons mounted at the rear of the vehicle to warn other vehicles approaching from the rear that an abnormally long vehicle is ahead.
One problem encountered by road users is that when travelling at night, while the rear reflectors and chevrons warn one that a heavy or abnormally long vehicle is ahead, they give no indication of the overall length of the vehicle. Attempts have been made to partly alleviate this problem by printing, in numerals or letters, the length of the vehicle at the rear of the vehicle but, in the absence of visible reference points, it remains difficult to judge the distance, particularly at night, and, consequently, hazardous to execute an overtaking manoeuvre.
The same difficulty can apply to the assessment of the length of a truck from the front and side.